The present invention relates to the field of magnetic resonance imaging. It finds particular application in conjunction with gradient coils for use therein.
Magnetic resonance imaging (MRI) is a widely used diagnostic imaging method. MRI equipment includes a magnet apparatus for generating a very strong, homogeneous static magnetic field. A so-called "open" magnet arrangement includes a pair of pole pieces disposed on opposite sides of an imaging region. Because it is necessary to impose a series of rapidly varying gradient magnetic fields on the static magnetic field, MRI systems typically include a set of gradient coils disposed between the pole pieces and the imaging region to generate the requisite gradients, usually in orthogonal x, y, and z directions.
Unfortunately, the time varying gradient fields induce eddy currents in electrically conductive materials, such as the magnet pole pieces, in the vicinity of the gradient coils. These eddy currents have various deleterious effects, most notably opposing rapid variations in the gradient magnetic fields. The interactions between the gradient magnetic flux and the iron pole pieces can cause field transients during changes in the gradients. The eddy currents also create a non-homogeneous magnetic field which decays with a relatively long time constant, for example on the order of hundreds of milliseconds. These effects lead to worsened image quality and can make imaging using relatively rapid sequences difficult, if not impossible.
Various attempts have been made to overcome these effects. In a so-called passive shielding technique, materials having desirable magnetic properties have been placed on the surface of the pole piece so as to attenuate the magnetic flux coupled to the bulk iron of the pole pieces. However, it is difficult to find materials having suitable properties, particularly at higher fields strengths. Because the magnetic flux generated by the gradients tends to penetrate deeply into the pole piece, the material needs to be relatively thick. Further, this approach decreases the undesirable effects but does not eliminate them completely.
Another technique involves electronic compensation of eddy current effects. Due to the complexity of the various interactions between the gradient fields and the pole pieces, effective compensation is difficult. Moreover, this technique does not account for gradient induced heating of the pole pieces.
Still another technique is disclosed in U.S. Pat. No. 5,555,251, entitled Arrangement to Minimize Eddy Currents in MR Imagers, which is commonly assigned with the present application. The disclosed technique includes uses a series of electrically insulated layers. A series of slots further reduces the eddy currents in the layers. While this technique has proven effective in reducing eddy current effects, further improvement is nonetheless desirable.